Across our codebase, we have a lot of images stored as URLs (we use filepicker.io). The default ActiveAdmin resources would have suited our purposes, but we wanted to display and be able to edit those images as images, without rewriting forms, show-views, and table columns for every single model we have! Since all the column names end in "img", I monkey-patched the ActiveAdmin internals to detect this, and to automatically place special columns that use filepicker.io's helpers (though you could do just about anything you want). So our resources barely need a config block!

All details about the extension points to use are in this gist, should be self-explanatory and applicable for many purposes:

https://gist.github.com/bpartridge/3a9f9cf77665cb25293c

Specifically, you can override:

    ActiveAdmin::Views::IndexAsTable.default_table
    ActiveAdmin::Views::AttributesTable.content_for
    ActiveAdmin::FormBuilder.default_input_type
